As is well known to those skilled in the art, cracked naphtha (obtained as product of a cracking operation or a coking operation) may contain a significant quantity of sulfur--up to as much as 13,000 wppm; and this material contributes a substantial quantity of undesired sulfur to the gasoline pool to which it is commonly passed. It is possible to decrease the sulfur content by (i) hydrotreating the whole feedstock to the cracking/coker unit or (ii) hydrotreating the product naphtha from these units.
The first noted alternative is a "brute force" effort that is very expensive in that it requires a large hydrotreater and it consumes significant quantities of hydrogen. The second-noted alternative is a more direct approach--but unfortunately it results in undesirable saturation of the olefins (typically originally present in amount of 20 v %-60 v %) and down to levels as low as 2 v %; and this reduces the octane number (Octane Number is the average of the Research Octane Number RON and the Motor Octane Number MON) of the product gasoline by as much as 10-20 units. Prior art desulfurization of full range FCC naphtha from 300 wppm down to 20 wppm of sulfur results in a typical decrease in octane number by about 14 units. This loss in octane number associated with desulfurization has a significant impact on the octane number of the refinery gasoline pool.
Typical prior art disclosures which are directed to hydrodesulfurization include:
U.S. Pat. No. 4,140,626 (Bertolacini and Sue-A-Quan) describes a selective hydrodesulfurization process employing a catalyst with a Group VIB metal and a Group VIII metal deposited on a support consisting of at least 70 wt % magnesium oxide (MgO). Preferably, the Group VI-B metal is molybdenum and the Group VIII metal is cobalt. Catalyst A (a catalyst of the invention of Bertolacini et al) contains 3 wt % CoO/.about.16 wt % MoO3 on a pure MgO support. Catalyst B is a sample of commercial Criterion HDS-2A CoMo on alumina hydrotreating catalyst (with similar levels of CoO and MoO.sub.3). Catalyst A is better than Catalyst B for hydrodesulfurization (HDS). In addition, catalyst A produces better octane numbers than Catalyst B at equivalent values of HDS (in the range of 75-85% HDS); however, the improvement is only .about.1.5 octane numbers. Surprisingly, for both catalysts, olefin saturation was fairly low (&lt;.about.40 wt %) and octane penalties were fairly insignificant (&lt;.about.2 octane numbers) for the ranges of HDS studied. Other catalysts of the invention (prepared on supports with at least 70 wt % magnesium oxide) give HDS improvements.
U.S. Pat. No. 4,132,632 (Yu and Myers) is very similar to the above described patent except that the metal loadings are restricted to 4-6 wt % for the Group VI-B metal and 0.5-2 wt % for the Group VIII metal. Again, preferably, the Group VI-B metal is molybdenum and the Group VIII metal is cobalt. Catalyst I (a catalyst of Yu et al) is .about.1 wt % CoO/.about.5 wt % MoO.sub.3 on a pure MgO support. Catalyst II contains .about.3 wt % COO/.about.17 wt % MoO3 on a support comprising 80 wt % MgO (i.e. a catalyst of U.S. Pat. No. 4,140,6626 supra). Catalyst I generally gives poorer HDS than Catalyst II, but Catalyst I gives less olefin saturation and better octane numbers at around the same level of HDS (.about.82-84%). The incremental octane improvement is small (.about.1.6 octane numbers). Again, for both catalysts, olefin saturation is fairly low (&lt;.about.40 wt %) and octane penalties are fairly insignificant(&lt;.about.2.6 octane numbers) for the ranges of HDS studied.
A paper entitled "DESULFURIZATION OF CAT CRACKED NAPHTHAS WITH MINIMUM OCTANE LOSS" presented at the 1978 NPRA Annual Meeting in San Antonio, Tex. by Coates, Myers and Sue-A-Quan sets forth a good overview of the development of what Amoco called their "Selective Ultrafining Process." The paper was presented about one year before the above described patents issued. The paper mentions two catalysts (presumably from the two patents). Sulfiding technique is mentioned as a major concern. The new catalysts show lower rates of deactivation than standard hydrotreating catalysts for HDS. Incremental octane improvements are said to be 4 MON and 4.5 RON at 90% HDS. The incremental octane improvements of the presentation were much larger than those shown in the subsequent Amoco patents.
GB 2,225,731 discloses hydrotreating catalysts comprising Group VI and Group VIII metal hydrogenation components on a support which comprises magnesia and alumina in a homogeneous phase. The mole ratio of Mg to Al is 3-10:1. The catalyst is said to have comparable HDS activity to similar catalysts based on alumina.
Additional background may be noted from:
(i) U.S. Pat. No. 3,539,306 to Kyowa Chemical Industry Co. as assignee of Kumura et al; PA1 (ii) U.S. Pat. No. 3,650,704 to T. Kumura et al; PA1 (iii) Cavani et al Anionic Clays with Hydrotalcite-like Structure as Precursors of Hydrogenation Catalysts Mat. Res. Soc. Extended Abstracts" (EA-24)--Pub by Materials Research Society; and PA1 (iv) O. Clause et al Preparation and Thermal Reactivity of Nickel/Chromium and Nickel/Aluminum Hydrotalcite-type Precursors Applied Catalysts 73 (1991) 217-236 Elsevier Science Publishers; PA1 (v) Eur. Pat. Application 0 476 489 Al to Haldor Topsoe A/S as assignee of E. G. Derouane et al; PA1 (vi) U.S. Pat. No. 3,705,097 issued Dec. 5, 1972 to Dow Chemical Co. as assignee of B. D. Head et al; PA1 (vii) U.S. Pat. No. 3,956,105 issued May 11, 1976 to Universal Oil Products as assignee of J. E. Conway. PA1 (viii) U.S. Pat. No. 4,962,237 issued Oct. 9, 1990 to Dow Chemical Company as assignee of D. E. Laycock.
The conventional catalysts for naphtha hydro-treating include CoMo, NiMo, NiW, CoMoP, and NiMoP metal oxides supported on gamma alumina typified by the commercial Criterion C-444 CoMo hydrotreating catalyst. Magnesia supported catalysts and silica-magnesia supported catalysts are disclosed in U.S. Pat. Nos. 2,853,429 and 3,269,938 respectively. The commercial BASF K8-11 catalyst, used in the water gas shift conversion, generally contains 4 wt % CoO and 10 wt % MoO.sub.3 on a magnesia-alumina-silica support. Contrary to the claimed advantages of the above-described Amoco patents and paper, one of the common drawback of catalysts on magnesia-containing supports is the low HDS activity compared to alumina (particularly gamma alumina) supported catalysts. It is commonly believed that the low surface area of magnesia-containing supports and the poor dispersion of MoO.sub.3 on magnesia-containing supports are the cause of the low HDS activities.
It is an object of this invention to provide a novel hydrodesulfurization process. It is another object of this invention to provide a magnesium-containing catalyst with a very high hydrodesulfurization activity. Other objects will be apparent to those skilled in the art.